The present invention relates generally to a method and apparatus for the melting and casting of silicon and silicon-containing materials, and is more particularly concerned with a method and apparatus for the continuous melting of silicon for use in semiconductor devices in order to reduce the time and expense associated with conventional melting and forming.
While the present invention will be primarily discussed herein with reference to the melting of silicon for use in semiconductor type devices, particularly photovoltaic cells, it will be understood that the applicability of the present invention is not thereby so limited.
In the conventional melting and forming of silicon for semiconductor use, it generally is the practice to charge a quartz vessel with pieces of silicon of various sizes and then heat the vessel under a protective gas blanket to a temperature sufficient to melt the silicon. The silicon is then formed into a monocrystalline rod by the Czochralski method, which is to seed the molten silicon and then slowly withdraw a formed rod from the molten silicon within the vessel. This method is generally quite time-consuming and requires expensive processing equipment.
In my copending patent application Ser. No. 751,342, now U.S. Pat. No. 4,256,681, filed Dec. 16, 1976, the entirety of which is hereby incorporated by reference, a method for forming silicon into a semicrystalline product is disclosed and claimed. In that method, silicon is melted within a melting vessel and then cooled under controlled conditions to produce a semicrystalline silicon product useful for the manufacture of photovoltaic cells. This method results in distinct cost advantages over the Czochralski method for producing silicon for photovoltaic cell uses.
However, several problems are associated with the above method for producing semicrystalline silicon for use in semiconductor devices such as photovoltaic cells. One problem is that it is often difficult to control the heating of the silicon to a temperature just exceeding the melting point of silicon of 1410.degree. C. In order to ensure that all of the silicon is melted, and because it is difficult to accurately control the heating of the vessel at such elevated temperatures, many times the silicon is heated to a temperature in excess of its melting point, e.g., the temperature of the silicon may reach 1450.degree. C. or more. Such heating of the silicon may cause harmful stresses within the vessel used to contain the molten silicon, may cause undesirable reactions between the silicon and the vessel, and increase the time necessary for the total operation of melting and forming the silicon.
In addition, because quartz and the silicon have different coefficients of thermal contraction, upon solidification of the silicon it is a common occurrence for the vessel to crack or even break, thereby rendering the vessel unfit for further use. Quartz vessels for melting silicon are quite expensive, and as a consequence breakage of the vessels adds considerably to the overall cost of the semiconductor device made from silicon by conventional methods.